One of the most meaningful aspects to human life is communication. The primary method of communication in humans is voice, thus impairment of the voice can have a devastating consequences in one's ability to fully participate in society. Adductor spasmodic Dysphonia (AdSD) is a type of focal dystonia that affects the muscles of the vocal cords and can severely impair the ability to speak. There is no diagnostic marker for this neurologic-based movement disorder and it is frequently confused with other voice disorders. Often people go years before being correctly diagnosed, only to learn that there are no long-term efficacious treatments available. Misdiagnosis of AdSD leads to provision of ineffective treatment such as voice therapy prolonging this debilitating communication disorder and resulting in unnecessary health care expenses. There is growing evidence that people with other types of focal dystonia have reduced cortical inhibition, leading to excessive excitation, which may contribute to the resultant abnormal motor control. Transcranial magnetic stimulation (TMS) is a safe, non-invasive tool that can provide extensive information about the neurophysiologic underpinnings in a disorder. This has been used extensively to investigate focal hand dystonia, greatly enhancing the understanding of that disorder, but has not been used in spasmodic dysphonia. This R21 proposal in response to PA 10-156 will use fine wire EMG electrodes in the muscle primarily affected by SD, the thyroarytenoid, and TMS to measure the central conduction latency and intracortical excitation and inhibition associated with the corticobulbar control of the larynx in healthy people and people with SD. It will also use well established techniques to collect the same data in the unaffected corticospinal control of the hand. Design: Two group comparison (healthy vs. AdSD) of the cortical excitability of the thyroarytenoid muscle and first dorsal interosseous muscle will be performed. Expected outcomes: It will refine techniques described in the preliminary data section to optimize procedures for TMS measurement in the larynx; begin to establish norms for the excitability measures in healthy people; determine the relationship of measures in the hand to the larynex; and compare data in subjects with SD to healthy. Impact: The results of this study will: (1) advance the knowledge regarding the central nervous system pathophysiology in AdDS, (2) elucidate potential neurophysiologic underpinnings of the disorder such as which intracortical mechanism differs from healthy and if it is localized to the affected musculature or pervasive. (3) It will refine techniques for other investigations of excitability in the larynx. Long term objecties: Future work may lead to a more definitive differential diagnosis between AdSD and other disorders and the development of innovative evidence-based interventions directly advancing the mission of NIDCD.